Control circuits are known for controlling brushless DC motors, such as, for example, regulating the rotational speed of brushless DC fan motors that cool the interiors of computers. One problem with brushless DC fan motors is that they traditionally have had a narrow usable input range. Fan speed and input current are approximately proportional to input voltages. Thus, if the input voltage from an unregulated source such as a battery were used to power a brushless DC fan, such as a typical 24 volt nominal battery, the voltage would vary from about 28 volts in float state to about 21 volts in discharged state. This change would cause a brushless DC fan rated at a nominal 3500 RPM to vary as much as about 1000 RPM over the above-mentioned range of battery voltages. Such a large variation in RPM means that the fan is not properly cooling a computer at the low-end of the RPM range, and that power is being wasted at the high-end of the RPM range.
Some brushless DC fan users have multiple input source voltages that their equipment is expected to operate from, with 24 volt and 48 volt systems being the most common. Such multiple source voltages pose the same problem in resultant RPM variation in a brushless DC fan motor as does a single input voltage source whose voltage level varies widely. Accordingly, there is a need to provide a brushless DC fan motor having a high input range with relatively little variation in motor rotational speed. For example, in the telecommunications industry, there is a need to provide a brushless DC fan motor having an input range of about 20-60 volts with little variation in motor rotational speed. However, other input voltage ranges may be provided for other motor applications.
Linear regulators have been used to regulate brushless DC fan rotational speed. However, the linear regulator approach poses an efficiency problem. A brushless DC fan that draws 18 watts at 21 volts will draw almost 27 watts when operating at 28 volts, and 54 watts at 56 volts input, with the increase in power draw having to be dissipated as heat.
Pulse width modulation ("pwm") has also been used in the prior art to regulate motor speed. One method commonly used is to pulse width modulate the commutation transistors to the brushless DC motor. This employment of pulse width modulation reduces the dissipation of energy involved with changing motor speed. However, pulse width modulating the commutation transistors does not permit large changes in input voltage without widely varying the rotational speed of the brushless DC motor. This method is most commonly used in thermal brushless DC fans to reduce brushless DC fan speed at low temperatures. The speed variation is unfortunately even wider than that of the non-speed controlled type, and clamp dissipation is still relatively high.
Another pwm approach is to use a full bridge driver. This involves placing a bipolar motor winding between the legs of four switching transistors and controlling the timing of the pwm modulator and commutation logic to regulate motor current. Wide input voltage ranges are possible with high efficiency. A well designed full bridge driver can regulate motor speed over a better than 3:1 range of input voltage. The chief drawbacks are complicated logic and the difficulties of driving the four switching transistors without cross conduction through the series connected pairs. Although many manufacturers offer integrated full bridge devices, most suffer from a limitation of current and/or voltage.
Another approach is to employ a pwm switching voltage regulator to accommodate a wide range of input voltages without widely varying the rotational speed of the motor. However, this requires relatively bulky filter inductors and capacitors.
Of the above-mentioned pwm approaches, the pwm voltage regulator regulates motor voltage. The other methods typically regulate motor current. Voltage regulation is preferred to minimize variations in desired brushless DC motor speed. In other words, the variation in motor speed from motor to motor for a given current is greater than the variation in motor speed for a given voltage. Additionally, motor torque is a function of motor current. Therefore, if motor current is reduced in order to reduce motor speed to a low value, the motor torque becomes low. This means that the motor speed is sensitive to applied load (i.e., back pressure). This sensitivity to back pressure results in large speed deviations from the desired value. Motor-starting at low desired speeds is also a problem in that if the motor current is set too low then the motor will not be able to overcome the magnetic detents used to position the rotor away from the null point. Unfortunately, controlling motor voltage while failing to control motor current to adhere to a symmetrical waveform has the potential to increase vibration and electrical interference.
Fans typically use one of two types of two-phase DC brushless motors, unipolar or bipolar. The difference between the two types is that a unipolar motor energizes two opposing poles of the four poles available, whereas a bipolar motor will energize all four poles at the same time, with the coils in quadrature having opposite magnetic polarity. Simply stated, the unipolar type uses two pairs of coils with one pair energized and the other pair not energized, with the poles always energized in the same polarity. The bipolar motor energizes the four poles at the same time with adjacent poles having opposite polarity. Rotation of the motor of the unipolar type is accomplished by alternating energized pairs, while the bipolar motor changes the polarity of the four poles.
The bipolar motor has double the output of the unipolar motor because all of the copper is utilized and all four poles act upon the magnet. Drive complexity is greater as the direction of current must be reversed rather than just interrupted. In both cases, however, a problem of asymmetrical current in the motor exists. The current in the motor windings is reversed twice for each complete revolution of the bipolar motor. Various factors influence or modify the symmetry of the motor such as the degree of magnet strength, offset in the position sensor, mechanical variations in the motor components, and variations in wire resistance. This causes the current levels and the waveform shapes to differ from each other within a rotational period and allow different torques to be applied to the rotor, increasing vibration and noise. Accordingly, it would be desirable to provide an apparatus and method which may correct such non-ideal behavior in both unipolar and bipolar motors.
It is also an object of the present invention to provide a brushless DC motor regulator which handles a relatively wide range of input voltages with little variation in the rotational speed of the motor.
It is another object of the present invention to provide a brushless DC motor regulator which controls motor armature current to a substantially symmetrical waveform.
It is a further object of the present invention to provide a brushless DC motor regulator that eliminates the relatively bulky filter capacitors and inductors interfacing the regulator and motor.
The above and other objects and advantages of the present invention will become more readily apparent when the following detailed description is read in conjunction with the accompanying drawings.